U. ROCHESTER / STONY BROOK (US) — Scientists have announced the first definitive observation of the transformation of muon neutrinos to electron neutrinos—a type of neutrino oscillation that had never been observed.

“In 1998, the discovery of neutrino oscillation in the atmospheric neutrinos by the Super-Kamiokande experiment led us to a new journey into the fascinating and mysterious world of neutrinos which is full of surprises,” says Chang Kee Jung, professor of physics at Stony Brook University, and international co-spokesperson for the T2K Collaboration.

“This discovery of electron neutrino appearance from muon neutrinos by the T2K experiment opens another critical door in our journey to unveil the secrets of our universe.”

Super Kamiokande is the worldʼs largest underground neutrino detector, and is located 1000 meters underground in Kamioka Mine, Hida, Gifu Precture, Japan. Super Kamiokande consists of a large cylinder 39.3 meters in diameter and 41 meters high that contains 50,000 tons of ultra-pure water. The inner walls of the cylinder are lined with about 11,200 photomultiplier tubes to detect Cerenkov light, which is emitted when a charged particle travels faster than the speed of light in water (this is three quarters of its speed in vacuum). (Credit: T2K)

The researchers report the new results this week at the European Physical Society meeting in Stockholm, Sweden.

The new finding is exciting to scientists because it could help explore a fundamental question of science—why is the universe made up almost exclusively of matter, when matter and antimatter were created in equal amounts in the Big Bang?

Somehow this balance changed over time to a dominance of matter. The mechanism for this to occur could lie in a process called charge-parity (CP) violation.

The experiment shows that researchers can now accurately observe the type of neutrino oscillation that will need to be studied in detail in future experiments aiming to measure CP violation, explains Steven Manly, professor of physics at the University of Rochester and part of the collaboration.

Until recently it was an open question as to whether or not it would be feasible to explore CP violation in neutrinos. CP violation has only been observed in another type of particle, quarks (for which Nobel Prizes were awarded in 1980 and 2008), never in neutrinos.

The observed type of neutrino oscillation is sensitive to CP violation and the T2K experiment will try to observe this process in neutrinos over coming years. It could be that the asymmetry between matter and antimatter lies with neutrinos, which is why observing CP violation in neutrinos would be exciting, Manly adds.

“Our goal now is to push to better understand the errors in the measurements and continue to collect sufficient data to explore this possible CP violation.”

One in a trillion

The T2K experiment, based in Tokai, Japan, expects to collect 10 times more data in the near future, including data with an antineutrino beam.

Manly explains that neutrinos are notoriously difficult to study, and the oscillation that the researchers seek can be mimicked by other processes. For that reason, the University of Rochester group has focused on understanding these other processes to ensure that what is measured is really the neutrino oscillation they have sought.

In 2011, the collaboration announced the first hints of this process. The researchers have now gathered 3.5 times more data and this transformation is firmly established. The probability that random statistical fluctuations alone would produce the observed excess of electron neutrinos is less than one in a trillion.

In the T2K experiment, a muon neutrino beam is produced in the Japan Proton Accelerator Research Complex, called J-PARC, located in Tokai village, Ibaraki Prefecture, on the east coast of Japan. The neutrino beam is monitored by a detector complex in Tokai and aimed at the gigantic Super-Kamiokande underground detector in Kamioka, near the west coast of Japan, 295 kilometers (185 miles) away from Tokai.

An analysis of the data from the Super- Kamiokande detector associated with the neutrino beam time from J-PARC reveals that there are more electron neutrinos (a total of 28 events) than would be expected (4.6 events) without this new process.

The current T2K collaboration consists of over 400 physicists from 59 institutions in 11 countries.

Funded by the US Department of Energy, Office of Science, the US T2K collaborating team includes Boston University; University of California, Irvine; University of Colorado; Colorado State University; Duke University; Louisiana State University; Stony Brook University; University of Pittsburgh; University of Rochester; and University of Washington (Seattle).